1. Field of the Invention
The present invention relates to communication system, and a method of transmitting data therein.
2. Description of the Related Art
Generally, a communication system has a channel wherein the communication environment is instantaneously changed in accordance with the position or moving speed of a user, and this change of the channel environment causes the whole communication environment to greatly deteriorate.
At present, the Internet is on an explosively increasing trend throughout the world. Accordingly, the standard of a radio data communication service for providing a data communication service to radio communication users is preparing. That is, the standard for an IMT-2000 system is now preparing as a new mobile communication system.
Meanwhile, the current radio communication environment is very deteriorated, and such deterioration limits the data transmission speed. Various kinds of methods have been used to overcome the deterioration of the environment and to provide a good quality of call required by the users.
One among the above methods is a method of adjusting a data rate. According to this data rate adjustment method, the data modulation type is selectively changed in a manner that a high-speed data transmission such as a quadrature amplitude modulation (QAM) is used in case that the communication environment is good, while a phase shift keying (PSK), which has a strong endurance against the interference noise, but has a low transmission speed, is used in case of an inferior communication environment.
Another is a method of effectively using limited radio channels by changing a coding rate in accordance with the communication environment. This coding rate changing method reduces an error rate by giving redundancy to information data.
According to the coding rate changing method, the coding rate is reduced by increasing the redundancy of data in case of the communication environment greatly deteriorated. By using this coding method, the data transmission speed is reduced. On the contrary, in case of a good communication environment, the coding rate is increased by reducing the redundancy of data. By using this coding method, the data transmission speed is increased.
According to the conventional data transmission methods for the communication system, the data transmission rate of a communication service is determined only by an amount of data to be transmitted without analyzing the channel environment of the user.
Accordingly, the call quality deteriorates in performing the communication service, and even a transmission delay is produced when the data having an error is retransmitted. As a result, a great loss is produced in the communication service.
Meanwhile, it is checked for each data frame by a cyclic redundancy check code whether an error is occurred in the data that is transmitted from a transmitting part to a receiving part through the radio communication environment.
This error checking method using the cyclic redundancy check code (CRC) detects a bit error with respect to all bits in one frame.
According to this method, the transmitting part considers the whole frame as a first number, and carries on a frame check sequence (FCS) of the transmitted data a remainder obtained by dividing the first number by a second number (for example, 100100000100010011) of a prearranged pattern with the receiving part.
The receiving part divides the number corresponding to the received frame by the second number having the same pattern, and compares the remainder thereof with the received frame check sequence (FCS). The bit error occurred within the received frame is detected in accordance with the result of comparison.
The error check method using the cyclic redundancy check code improves the reliability of bit error detection by increasing the sequence of bit pattern in case that the state of the communication environment is inferior.
Meanwhile, if an error is occurred, the receiving part can request to the transmitting part re-transmission of the frame in which the error is occurred.
In order to reduce the error rate of the frame subject to re-transmission, the receiving part soft-combines the re-transmitted data with the stored data that was firstly transmitted. This improves the error rate of the data transmitted to the receiving part.
However, a memory is essentially required to use the soft combining method. If an error is occurred again in the retransmitted data, an additional memory capacity is required. In performing the re-transmission of the data, a data transmission delay is also produced.
Therefore, an object of the invention is to solve the problems involved in the related art, and to provide communication system and a method of transmitting data therein that can improve the transmission efficiency by determining the data transmission rate in accordance with the channel state of a receiving part.
It is another object of the present invention to provide communication system and a method of transmitting data therein that can improve the receiving probability of data re-transmitted from a transmitting part when a receiving part requests the data re-transmission.
It is still another object of the present invention to provide communication system and a method of transmitting data therein that can reduce the required capacity of memory by preventing an error from repeatedly occurring in the transmitted data.
In one aspect of the present invention, a receiving part measures a receiving quality of a signal received from a transmitting part, and determines the data transmission rate to be used in a transmitting part by the measured receiving quality. The transmitting part differently applies a data coding rate or modulation type of data to be transmitted in accordance with the determined data transmission rate, and transmits the data to the receiving part.
Preferably, the data transmission rate may be determined as the data transmission rate mapped on the receiving quality measured from a mapping table prepared by a fixed type algorithm having a receiving quality range that is fixed in accordance with the data transmission rate of the received signal.
Preferably, the data transmission rate may be determined as the data transmission rate mapped on the receiving quality measured from a mapping table prepared by a variable type algorithm having a receiving quality range that varies in accordance with the data transmission rate of the received signal.
In the mapping table according to the fixed type algorithm, the receiving quality range that belongs to a certain data transmission rate is divided into predetermined steps. Accordingly, if a frame(or CRC) error is occurred in the received signal in the receiving part, the step corresponding to the measured receiving quality is heightened by one step. Meanwhile, if a frame(or CRC) error is not occurred in the received data in the receiving part, the step corresponding to the measured receiving quality is lowered by one step. Then, a data transmission rate on the mapping table that corresponds to the heightened or lowered receiving quality is provided to the transmitting part as a determined data transmission rate.
It is assumed that in the variable type algorithm, the heightened interval of the receiving quality is defined as xe2x80x9cxcex94xe2x80x9d, and the lowered interval of the receiving quality is defined as xe2x80x9cxcex93xe2x80x9d. If a frame(or CRC) error is occurred in the received data in the receiving part, the mapping table is changed in a manner that the receiving quality range is lowered by xcex93xe2x80x9d. Meanwhile, if a frame(or CRC) error is not occurred in the received data in the receiving part, the mapping table is changed in a manner that the receiving quality range is heightened by xe2x80x9cxcex94xe2x80x9d. Then, a data transmission rate that is mapped on the newly measured receiving quality range is provided from the changed mapping table to the transmitting part.
Preferably, in order to measure the receiving quality, one among a pilot signal level, signal to interference ratio, bit error rate, and frame error rate of data received in the receiving part may be used.
Preferably, if the received data in the receiving part is error, the transmitting part may assign more power to the data, and re-transmit the resultant data to the receiving part. Also, the transmitting part may re-transmit the data with the data transmission rate determined by the fixed type or variable type algorithm.